1. Field of the Invention
The invention relates to a transparent coating or film comprised of a plastic material, and to safety glass panes covered with such a coating or film and employed particularly on means of transport, and in particular the invention relates to improvement in the performance of such panes in the presence of hazards of condensation and fogging.
2. Description of the Prior Art
Certain safety glass panes are being increasingly used, in large sizes in vehicles, vessels and buildings, and in small sizes in smaller windows, masks, lenses, etc. These panes comprise a monolithic or laminated support comprised of glass or a plastic material and covered, by the intermediary of an adhesive layer, by a coating or film of a transparent, "anti-laceration" plastic material, i.e. a coating or film which, in the event of breakage of the glass pane, covers the sharp edges of the glass fragments and prevents possible injury to passengers due to these sharp edges. The said plastic coating or film covering is also self-healing, i.e. it is comprised of a plastic material wherein local impressions or scratches (abrasive impressions) on the surface disappear spontaneously in a short time. Such glass panes are described, e.g., in Fr. Pat. Nos. 2,187,719, 2,316,913, 2,320,563, and 2,398,606. Such panes may comprise, e.g., a laminated glass pane comprised of two glass sheets joined by an intercalated sheet of a plastic material such as polyvinyl butyral and covered by a coating or film of a transparent plastic material essentially comprised of a heat-setting polyurethane.
Automobile windshields in particular may be subjected to sudden changes of temperature and humidity. After the surface temperature of the windshield falls below the dewpoint of the ambient air, water condensation occurs, which can impede the driver.
The effects of the fogging phenomenon vary appreciably depending on the type of transparent surface. On bare glass, the droplets of water spread out much more readily than on an organic film. By simple wiping of the bare glass, a water film is formed which is relatively regular and transparent from the optical standpoint, and the presence of this water film itself substantially attenuates the adverse optical effects of the condensation. In general, where the surface comprises a film of a plastic material, the droplets of condensed water may be much more prominent and dense; this phenomenon varies over a wide range depending on the nature of the film. In this case, simple wiping action will remove the droplets but cannot form a water film, and the fogging may recur even more quickly than it developed originally.
Various solutions have been proposed in the past to ameliorate the fogging behavior of transparent surfaces of glass or plastic material.
The general technique is known of applying hydrophilic agents to the transparent surface, which agents can mix with the water when there is a fogging hazard, to form a homogeneous thin film. For example, one may appreciably improve the antifogging properties of a surface by applying a film of compounds of (ethylene) glycol or compounds of glycerin. However, these watersoluble compounds are removed as soon as wiping occurs, and thus need to be resupplied perpetually, which is inconvenient and costly.
German Pat. No. 1,928,409, for example, discloses other antifogging treatment methods for transparent surfaces, said methods comprising the application of films of polyacrylate and/or polymethacrylate compounds with free hydroxyl groups. These very hydrophilic films absorb water without being dissolved themselves. As a result of the absorption of water these films swell more or less substantially; further, they soften and become more sensitive to abrasion which eventually has an adverse effect on the transparence of the surface treated.
Thus, when one employs antifogging treatments comprising the application of antifogging surface films, one inevitably faces the dilemma of conflicting requirements: Improved antifogging action reduces abrasion resistance, while improved abrasion resistance corresponds to lessened antifogging action. One seeks means of improving the abrasion resistance of the films by introducing a hardening agent into their composition. However, in general such hardening agents achieve their effect by polycondensation with the polymers which comprise the film. In so doing they render unavailable the hydroxyl-type groups which confer on the film the hydrophilic properties which promote antifogging action.
In the case of glass pane covered with a coating or film of palstic material which has antilaceration and self-healing properties, e.g. a pane such as described in one or more of French Pat. Nos. 2,187,719, 2,316,913, 2,320,563, and 2,398,606, the fogging problem, while diminished in comparison to that with a classical monolithic or laminated glass pane, is nonetheless present, and is posed in terms which may be more complex: A treatment may not under any conditions diminish the excellent antilaceration and self-healing properties which are conferred by the said coating or film of transparent plastic material; the treatment must be effective over a long period of time; and the treatment may not modify the nature of the coating or film or plastic material in such a way as to carry the risk of detriment to other properties which said coating or film confers on the said glass pane.